This week, Marco Rubio received a boost that may be just as important as the glowing press coverage he has received since the last Republican debate. Cory Gardner, the freshman senator from Colorado, announced Monday that he would back Rubio in the 2016 primary race, just two days after Paul Singer—an influential Republican donor with billions at his disposal—did the same.

For Rubio, these endorsements could have a dramatic effect, rallying donors, party leaders, and voters behind him in the final months before the Iowa caucuses. With their backing, Rubio could make a serious play to become the establishment pick for the nomination.

“These endorsements are very important in providing a signal to other people in the party that Rubio is the man to beat, that he has the best chance to break free of the pack,” said Martin Cohen, a professor of political science at James Madison University, in an interview. 

But the latest developments also highlight a divergence in when these endorsements matter. While they remain critically important for underdog candidates like Rubio—and played a big role in Barack Obama's insurgent campaign in 2008—endorsements hold very little sway in the current Democratic primary. Hillary Clinton has already shored up more than enough support to demonstrate that the Democratic establishment backs her. This was painfully evident when Bill de Blasio, the liberal mayor of New York City, gave his endorsement to Clinton after months of holding it out to entice more progressive proposals from the candidate. Clinton's team announced the endorsement in a round-up of 87 mayors who had backed her campaign.


We are in the midst of what political scientists call “the invisible primary,” the year before the Iowa caucuses when candidates compete for campaign operatives, donors, and endorsements. Behind closed doors, party leaders are weighing the candidates, looking for the one to unite their party.

In their 2008 book The Party Decides, Cohen and his coauthors David Karol, Hans Noel, and John Zaller found that endorsements are often the best predictor of the eventual nominee. Using data from elections between 1980 and 2004, they demonstrated that “early endorsements in the invisible primary are the most important cause of candidate success in the state primaries and caucuses."

Their findings are somewhat counterintuitive. Sweeping electoral reforms during the 1970s were intended to give voters the power to pick their nominee, wresting the nominating process away from party strongmen like William “Boss” Tweed, who ran New York politics for decades. However, The Party Decides demonstrated that political parties still wield enormous power in primaries, effectively selecting a frontrunner months before voters make it to the caucuses or ballot boxes.

As a result, the media keeps detailed tallies of endorsements collected by each candidate in the hopes that they predict the eventual winner. Throughout the summer, Rubio was something of an afterthought in the political press, after underperforming in both polls and fundraising tallies. But when Gardner on Monday appeared on Fox News—announcing that “Marco Rubio presents this nation with the greatest possibilities and opportunities to meet the challenges of this generation”—it signaled change was afoot.

“Endorsements early on can really pick somebody up who may not be doing that well in the polls and vault him into a good position,” Cohen said.

Gardner carries particular weight among Republicans. He is considered a rising star in Republican ranks, after achieving a razor-thin victory over Senator Mark Udall of Colorado last year. At 41, he could help cement the idea that Rubio is at the vanguard of the next generation of Republican leaders.   

“Singer can obviously help financially. Gardner can be helpful in Colorado, and in reinforcing the image of Rubio as the choice of a new generation,” said David Karol, an associate professor at the University of Maryland, in an interview. “But beyond what help they can provide directly, they are also important in that they stepped up. Many people are scared to be the first to make a move. It will probably be easier for others to follow.”

“There could be a bandwagon effect,” Cohen said. “In the next few weeks, he could go to the top of the heap.”

With their support, Rubio could catch up to Bush, who commanded an early lead in endorsements. According to a point system that FiveThirtyEight uses to track endorsements, Bush has 37 points, while Rubio has 13.


Compare that margin to the figures from the Democratic primary. Clinton is practically untouchable when it comes to endorsements. According to FiveThirtyEight, she has already amassed 391 points. Bernie Sanders has two.

“She is clearly established as the most formidable frontrunner we have had in the last 40 years,” Cohen said. “Something very significant would have to happen—maybe a real scandal—for the party leaders to abandon her now.”  

This is bad news for Bill de Blasio, who has worked all year to fashion himself as a progressive leader along the lines of Elizabeth Warren. On “Meet the Press” in April, just hours before Clinton formally announced her candidacy, de Blasio told host Chuck Todd that he would wait to endorse her until he saw the details of her economic plan. “We need to see the substance,” he said.

In the six months since then, it has become clear that de Blasio lacks the national stature to make a palpable impact on the race. “Hillary Clinton is in a very strong position already,” Karol said. “De Blasio is not going to give her credibility—she already has credibility. Most people in Iowa don’t know who he is.” 

With Clinton riding a wave of support after a successful debate performance in Las Vegas and her Benghazi testimony, de Blasio has had to back away from his efforts to nudge Clinton to the left. “He was trying to become this national progressive leader, but that gambit has not worked,” Karol said. “He climbed out on a limb and has to climb back to a safe place.”

Still, his endorsement does have some significance. It demonstrates that Clinton has successfully begun to unite her party after a summer when its liberal wing looked like it might pose a real fight. And it also shows that single endorsements are virtually meaningless now.

“Hillary Clinton is already the overwhelming favorite of the Democratic Party elite,” Karol said. “De Blasio is not even the icing on the cake, he's more like one sprinkle.”